It depends how you count those birds. If you have to give up one that you already have for the two that you might catch on the bush (which is how I read the saying) - you will get a gist of "opportunity cost" - no?
But again, my problem is not with economic modeling but with economic modeling turned into ruling class ideology. It is the same as "marxism-lenininsm" in fSU, or geo-centrism in the middle ages.
Wojtek